Licence: Public Domain Mark
Credit: Color-blindness : remarks / by B. Joy Jeffries. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![[Dr. Jeffries then described the worsted test, referring- to his Man- ual for more detailed explanation. Its seeming simplicity was dwelt on, and the consequent danger of any one not most thoroughly posted both in theory and practice, making most fatal mistakes, finding color- blindness where it did notreally exist, and not detecting it where present. The test was most perfect, but never could be understood too well. He described how ridiculously it had been misused by railroad officials and others.] Dade's test is copied from Holmgren’s. On a card in book form are arranged rows of little squares of worked worsteds. Some rows are correct, namely, arranged from the darker to lighter of the same color. Others are arranged of the colors the color-blind especially confound,, and they will say when asked that these rows are correct. It is arranged to detect red, green, and violet-blindness. Bonders' worsted test consists, as you see, of a series of little pieces of wood, on which are wound colored worsteds. By testing a large number of color-blind, he found just what colors they confounded, and he winds one of them on the stock, letting the other color come in as a single thread once or twice in the row. Now, these two colors looking alike to the color-blind, they cannot see any difference, and cannot say that there are any or how many bands of a different color or appear- ance from the rest of the stock. The sticks, as I show, are arranged to detect the three forms of color-blindness—red, green, and violet-blind- ness. The quantitative measure of color-hlindness is very important. The defect exists in all degrees; and, as a certain amount may not prevent persons acting in some capacities, the exact determination of the exist- ing chromatic loss becomes very necessary. For this purpose various methods have been devised, based upon the several principles of color- perception which have been described or alluded to. These are in the Connecticut rules. Holmgren's method, icith colored shadows, I regret not being able to exhibit to you, as the apparatus is too cumbersome to be easily trans- ported. It is described in my United States Manual. In the centre is a round-wicked petroleum lamp; from the stand project two horizontal arms; on one is the colored glass before the light, the pencil to cast a shadow, and the white porcelaine on which it is cast; on the other arm is the mirror, by reflection from which another shadow is cast on the porcelaine; the mirror is movable towards and from the lamp. At a certain distance the shadows will appear of the same brightness to the normal eye, and we measure the amount of color-blindness by the differ- ence in distance of lamp and mirror requisite to cause*the two shadows to seem equally bright. This is one of the “control methods” used by Professor Holmgren on the Swedish railroads. Bonders' method of quantitatively determining the color perception by reflected light, is by placing these colored disks,- from one to twenty millimetres in diameter, on a surface of black velvet hung on the wall. On approaching slowly a small colored object, the normal eye detects the color but little later than he sees the light. The person who has any loss of the chromatic sense, that is in any degree color-blind sees the color much later than he sees the reflected light. On this is based the method of examination. Bright saturated colors, one millimetre in diameter, when placed on black velvet in a good light, will be seen by an eye with normal visual acuteness at a distance of five metres.](https://iiif.wellcomecollection.org/image/b22370110_0014.jp2/full/800%2C/0/default.jpg)
